1) Field of the Invention
The invention relates to a method of manufacturing a 3D micromirror, and more particularly, to a method of manufacturing a 3D micromirror using a silicon micro-machining process.
(2) Description of the Prior Art
Micromirror devices based on MEMS technology had a humble debut in the late eighties as display devices. However, a spurt in research activities took place in the mid to late nineties after they were identified as most promising candidates for futuristic all-optical communication networks. There are two basic configurations for micromirror arrays based on MEMS technology, namely 2D in-plane and 3D free space out of plane. This division is based on MEMS silicon process technology. The 3D free space switching array is more efficient than the 2D configuration because it requires a smaller number of mirrors for a similar cross switching function. However, it is more challenging from a packaging and fiber coupling alignment point of view. The requirement for other parameters such as surface reflectivity, curvature, switching speed, cross talk, etc., is similar in both configurations.
There has been some recent work on 3D free space MEMS micromirror technology. Single crystal silicon material is a natural choice for high reliability, very good polished surface, and better flatness. U.S. Pat. No. 6,563,106 to Bowers et al and U.S. Pat. No. 6,556,737 to Miu et al disclose mirrors fabricated from thick single crystal silicon and actuators fabricated from thin single crystal silicon. The actuators of Bowers et al are electrostatic and parallel plate actuators requiring high voltage. Miu's actuators are electromagnetic. U.S. Pat. No. 6,504,643 to Peeters et al has a single crystal silicon mirror and MoCr electrostatic and parallel plate actuators requiring high voltage. U.S. Pat. No. 6,480,320 to Nasiri describes thick single crystal silicon mircomirrors and silicon-on-insulator (SOI) single crystal silicon electrostatic and parallel plate actuators requiring high voltage. Other materials can be used to fabricate micromirrors. For example, U.S. Pat. No. 6,386,716 to Hagelin et al shows polysilicon micromirrors and electrostatic actuators requiring high voltage.
The article “Micromirrors for Adaptive-optics Arrays” by Michael A. Helmbrecht et al, Transducers '01 Eurosensors XV, June 2001, describes micromirrors built using wafer bonding techniques. The article “Three-dimensional structures obtained by double diffusion and electrochemical etch stop” by S. Marco et al, Journal of Micromech. Microeng. 3 (1993) pp. 141-142, shows a two-step silicon layer method of forming non-uniform diaphragms and bridges. This disclosure does not anticipate using the two-step silicon method to build a 3D micromirror device. There is no recognition of the need to prevent breakage by cutting thick silicon having thinner edges.